Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Multiple-input multiple-output (MIMO) communication systems employ multiple transmit antennas and multiple receive antennas to communicate data symbols over a communications channel. MIMO communication systems may allow a plurality of mobile stations to be serviced utilizing a same frequency band. In this manner, MIMO communication systems may advantageously increase an amount of data the communication systems is able to send to users.
MIMO systems may find use in a variety of applications including, but not limited to, wireless networks, cellular systems including 3G and 4G systems, such as 3GPP LTE-Advanced, local and wide area networks, and wireless broadband systems (such as WiMAX).
Generally, a transmitter in a MIMO system may have a plurality of antennas. Receiving devices may each have a single antenna for receipt of data over the communications channel, or may have multiple antennas.
The communications channel, however, may introduce a variety of non-idealities to a transmitted signal, such as may be caused by multipath interference, reflections, motion of one or more mobile stations, or other properties of a communications channel. The signals received by a receiver over the communications channel may be related to the transmitted signals by a channel matrix H. The channel matrix generally refers to a matrix of data which may represent the operation of the communications channel on the transmitted data symbols, including representations of such effects as reflections.
Orthogonal frequency division multiplexing (OFDM) modulation techniques may be used to modulate data for transmission over a communications channel. OFDM generally involves modulating different portions of the data to be transmitted using different sub-carrier frequencies.
Block coding techniques may be used in communication systems to encode data and transmit multiple data streams over a communications channel, which may improve the reliability of data transfer. Block coding techniques include space-time block coding, which may transmit multiple data streams across multiple antennas. Alamouti space time block coding is one particular type of space-time block coding known in the art. Space-time block coding techniques may require that the communications channel be constant during consecutive data transmission times, which may not always be advantageous. Another type of block coding technique is space-frequency block coding. Alamouti space-frequency block coding is one particular type of space-frequency block coding known in the art. Space-frequency block coding techniques may require that different frequencies experience similar communications channel properties, which may be more advantageous in some cases.
Block coding techniques may commonly be used in conjunction with OFDM. Decoding of block encoded OFDM transmissions may require knowledge of properties of the communications channel over which the transmission is received. Accordingly, systems utilizing block encoded OFDM transmissions generally measure one or more properties of the communications channel, such as the channel matrix. One way to measure properties of the communications channel is to insert pilot symbols into the transmitted data streams. The pilot symbols may be known to the receiver and the known pilot symbols may be compared with the received signals to generate an estimate of one or more properties of the communications channel. The need to insert pilot symbols into the transmitted data streams may adversely effect a data rate attainable in the communications system.